我用这种方法解决了我的问题。我已经从前一个类传递了getApplicationContext，它已经访问了getApplicationContext。我已经将Inputstream对象传递给我的新类营养素。

try{
                    InputStream is= getApplicationContext().getAssets().open("nutrient_list.json");
                    Nutrients nutrients=Nutrients.getNutrients(topRecognition,is);

                  } catch (IOException e) {
                    e.printStackTrace();
                  }

那些仍然有问题的onAttach(活动活动)，它只是改变了上下文-

    @Override
public void onAttach(Context context) {
    super.onAttach(context);
    this.context = context;
}

在大多数情况下，保存上下文对您来说就足够了——例如，如果您想执行getResources()，您可以直接从上下文执行。如果你仍然需要将上下文放入你的活动中，那么就这样做吧

 @Override
public void onAttach(Context context) {
    super.onAttach(context);
    mActivity a; //Your activity class - will probably be a global var.
    if (context instanceof mActivity){
        a=(mActivity) context;
    }
}

由user1868713建议。

我用这种方法解决了我的问题。我已经从前一个类传递了getApplicationContext，它已经访问了getApplicationContext。我已经将Inputstream对象传递给我的新类营养素。

try{
                    InputStream is= getApplicationContext().getAssets().open("nutrient_list.json");
                    Nutrients nutrients=Nutrients.getNutrients(topRecognition,is);

                  } catch (IOException e) {
                    e.printStackTrace();
                  }

Commit调度事务，也就是说，它不会立即发生，而是在下一次主线程准备就绪时在主线程上进行调度。

我建议加一个

onAttach(Activity activity)

方法，并在它上面放一个断点，看看它什么时候相对于你对asd()的调用被调用。您将看到它在您调用asd()退出的方法之后被调用。onAttach调用是片段附加到其活动的地方，从这一点上getActivity()将返回非空(注意还有一个onDetach()调用)。

我正在使用OkHttp，我刚刚遇到了这个问题。

@thucnguyen的第一部分是正确的。

这发生在您在另一个线程中调用getActivity()时，该线程在片段被删除后结束。典型的情况是在HTTP请求完成时调用getActivity()(例如Toast)(例如onResponse)。

一些HTTP调用甚至在活动关闭后仍在执行(因为完成HTTP请求可能需要一段时间)。然后，我通过HttpCallback试图更新一些片段字段，并在尝试getActivity()时得到一个空异常。

http.newCall(request).enqueue(new Callback(...
  onResponse(Call call, Response response) {
    ...
    getActivity().runOnUiThread(...) // <-- getActivity() was null when it had been destroyed already

IMO的解决方案是防止当片段不再活跃时发生回调(不仅仅是Okhttp)。

解决办法:预防。

如果你看一下片段的生命周期(更多信息在这里)，你会注意到有onAttach(Context Context)和onDetach()方法。它们分别在Fragment属于一个活动之后和停止之前被调用。

这意味着我们可以通过在onDetach方法中控制它来防止回调的发生。

@Override
public void onAttach(Context context) {
    super.onAttach(context);

    // Initialize HTTP we're going to use later.
    http = new OkHttpClient.Builder().build();
}

@Override
public void onDetach() {
    super.onDetach();

    // We don't want to receive any more information about the current HTTP calls after this point.
    // With Okhttp we can simply cancel the on-going ones (credits to https://github.com/square/okhttp/issues/2205#issuecomment-169363942).
    for (Call call : http.dispatcher().queuedCalls()) {
        call.cancel();
    }
    for (Call call : http.dispatcher().runningCalls()) {
        call.cancel();
    }
}

The other answers that suggest keeping a reference to the activity in onAttach are just suggesting a bandaid to the real problem. When getActivity returns null it means that the Fragment is not attached to the Activity. Most commonly this happens when the Activity has gone away due to rotation or the Activity being finished but the Fragment still has some kind of callback listener registered preventing it from being garbage collected. When the listener gets called if you need to do something with the Activity but the Activity is gone there isn't much you can do. In your code you should just check getActivity() != null and if it's not there then don't do anything. If you keep a reference to the Activity that is gone you are preventing the Activity from being garbage collected. Any UI things you might try to do won't be seen by the user. I can imagine some situations where in the callback listener you want to have a Context for something non-UI related, in those cases it probably makes more sense to get the Application context. Note that the only reason that the onAttach trick isn't a big memory leak is because normally after the callback listener executes it won't be needed anymore and can be garbage collected along with the Fragment, all its View's and the Activity context. If you setRetainInstance(true) there is a bigger chance of a memory leak because the Activity field will also be retained but after rotation that could be the previous Activity not the current one.